Reaching is an essential activity of daily living, the loss of which imposes immediate and significant disability. Successful reaching can be accomplished through a variety of joint and muscle combinations on different repetitions, even when reaching along an identical hand path. This variability or redundancy of motor solutions ('motor redundancy') forms the basis of the famous 'degrees-of-freedom problem' in movement science. Nonetheless, to what extent the nervous system utilizes motor redundancy during the performance of functional tasks is still unclear. In addition, it is unclear whether limited use of motor redundancy underlies the dysfunctional reaching displayed by neurological patients. A goal of our laboratory has been to understand the extent to which motor redundancy is utilized to control the arm during reaching and how the use of motor redundancy is altered in persons with motor dysfunction resulting from stroke. The specific aims of this application are to (1) characterize differences in the synergies of joint motion of patients with mild to moderate impairments and healthy individuals and how these relate to the use of motor redundancy for reaching, (2) determine the contribution of motor planning to the use of motor redundancy, (3) account for differences in motor redundancy with respect to posture and movement timing, and (4) develop a formal model of reaching control to account for current and predict experimental results. These aims will be addressed through three experiments involving healthy individuals and persons with a stroke, combined with formal modeling and simulations. The Uncontrolled Manifold (UCM) approach, used to identify motor redundancy, will be combined with principal components analysis to characterize synergies of joint motion (1) when reaching within and beyond subjects' functional arm length; (2) when reaching under a timed response paradigm (to address motor planning effects); and (3) when reaching with and without an external timing constraint. The UCM approach will be combined with principal components analysis in this application to more fully explore movement synergies of patients with stroke in comparison to matched control subjects and will be extended to an analysis of motor redundancy with respect to movement timing. Simulations of our mathematical model, with manipulations of factors such as oscillator stability or relative inter-joint coupling strength, will help to account for differences in the use of motor redundancy to control posture and movement timing between patients with mild and moderate impairments and healthy individuals, and to generate predictions for future experimental results.